Considerable attention has focused on stem cells such as embryonic stem cells and non-embryonic stem cells, and their uses in a range of therapies. The availability of stem cells from non-embryonic tissues can greatly contribute to cell replacement therapies such as bone marrow transplants, gene therapies, tissue engineering, and in vitro organogenesis. Production of autologous stem cells to replace injured tissue would also reduce the need for immune suppression interventions. Beyond their potential therapeutic applications, homogenous preparations of, for example, adult stem cells would have another important benefit, the ability to study their molecular and biochemical properties.
The existence of stem cells in somatic tissues is well established by functional tissue cell transplantation assays (Reisner et al., 1978). However, their individual identification has been difficult to accomplish. Even though their numbers have been enriched by methods such as immuno-selection with specific antibodies, there are no known markers that uniquely identify stem cells in somatic tissues (Merok and Sherley, 2001). Secondly, adult stem cells are often present in only minute quantities, are difficult to isolate and purify, and their numbers may decrease with age.
Mammalian adult stem cells replicate by asymmetric self-renewal to replenish cells in tissues that undergo cell turnover but maintain a constant cell mass (J. L. Sherley, Stem Cells 20, 561 (2002); M. Loeffler, C. S. Potten, in Stem Cells (ed, Potten, C. S.) 1-27 (Academic Press, London, 1997)). Each asymmetric adult stem cell division yields a new stem cell and a non-stem cell sister. The non-stem cell sister becomes the progenitor of the differentiated cells responsible for mature tissue functions (Loeffler, 1997; Sherley, 2002). In contrast, embryonic stem cells exhibit symmetric self-renewal (Stead E, et al., Oncogene 21(54):8320-33 (2002); Savatier P, et al., Oncogene (3):809-18 (1994)).
Cells display a range of expression states at certain times or in response to environmental stimuli, e.g. from resting to replicating. Recently attention has focused on identifying gene patterns, including mRNA patterns and protein expression patterns, connected with such different states. This is sometimes referred to as gene profiling—where transcriptomes associated with a specific state are identified. Being able to identify certain genes (and/or associated proteins and/or transcripts) that are associated with a cell being in a specific state permits one to readily identify and screen for specific cells, even from a population of related cells.
Thus, despite the need for methods to identify and isolate specific cells from an individual, it has not been possible to readily do so. Accordingly, it would be desirable to have a method to identify markers associated with different cells and/or different cells states in mammalian tissues.